Low-frequency Lorentz marine seismic source

ABSTRACT

This disclosure is related to marine seismic sources, for example marine seismic sources known in the art as benders. Some embodiments of this disclosure use Lorentz forces to produce seismic energy. For example, magnets and wire coils may be attached to one or more plates of a marine seismic source, and the Lorentz interaction between them may cause deformation of the plates to produce seismic energy. Such marine seismic sources may be components of a marine seismic survey system, and may be used in a method of marine seismic surveying. Methods of making marine seismic sources are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Provisional Patent ApplicationNo. 61/920,118, filed Dec. 23, 2013, which is hereby incorporated byreference in its entirety.

BACKGROUND

In the oil and gas exploration industry, marine geophysical prospectingis commonly used in the search for hydrocarbon-bearing subterraneanformations. Marine geophysical prospecting techniques may yieldknowledge of the subsurface structure of the Earth, which is useful forfinding and extracting hydrocarbon deposits such as oil and natural gas.Seismic surveying is one of the well-known techniques of marinegeophysical prospecting.

In some instances of seismic surveying conducted in a marine environment(which may include saltwater, freshwater, and/or brackish waterenvironments), one or more marine seismic sources are typicallyconfigured to be submerged and towed by a vessel. The vessel istypically also configured to tow one or more laterally spaced streamersthrough the water. At selected times, control equipment may cause theone or more marine seismic sources to actuate. Seismic signals may thenbe received by sensors disposed along the streamers. Data collectedduring such a seismic survey may be analyzed to assist identification ofhydrocarbon-bearing geological structures, and thus determine wheredeposits of oil and natural gas may be located.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of a marine seismic source according tothis disclosure.

FIGS. 2A-2B illustrate a disassembled view of an embodiment of a marineseismic source according to this disclosure.

FIG. 3 illustrates a cross-sectional view of an embodiment of a marineseismic source according to this disclosure.

FIG. 4 illustrates a cross-sectional view of an embodiment of a portionof a marine seismic source according to this disclosure.

FIGS. 5A-5B illustrate a disassembled view of an embodiment of a marineseismic source according to this disclosure.

FIG. 6 illustrates an embodiment of a marine seismic source according tothis disclosure.

FIG. 7 illustrates an embodiment of a marine seismic source according tothis disclosure.

FIG. 8 illustrates an embodiment of a method according to thisdisclosure.

DETAILED DESCRIPTION

This specification includes references to “one embodiment” or “anembodiment.” The appearances of the phrases “in one embodiment” or “inan embodiment” do not necessarily refer to the same embodiment.Particular features, structures, or characteristics may be combined inany suitable manner consistent with this disclosure.

Various devices, units, circuits, or other components may be describedor claimed as “configured to,” “usable to,” or “operable to” perform atask or tasks. In such contexts, “configured to,” “usable to,” and“operable to” are each used to connote structure by indicating that thedevices/units/circuits/components include structure that performs thetask or tasks during operation. As such, thedevice/unit/circuit/component can be said to be configured to, usableto, or usable to perform the task even when the specifieddevice/unit/circuit/component is not currently operational (e.g., is noton or in operation). The devices/units/circuits/components used with the“configured to,” “usable to,” or “operable to” language may includeelectronic hardware—for example, circuits, memory storing programinstructions executable to implement the operation, etc.—mechanicaldevices, or other types of structure. Reciting that adevice/unit/circuit/component is “configured to,” “usable to,” or“operable to” perform one or more tasks is expressly intended not toinvoke 35 U.S.C. § 112(f), for that device/unit/circuit/component.

In some embodiments, various items of information relating to seismicsurveying may be embodied in a geophysical data product. A “geophysicaldata product” may be stored on a computer-readable, non-transitorymedium and may embody geophysical data (such as raw streamer data,processed streamer data, two- or three-dimensional maps based onstreamer data, etc.). Some non-limiting examples of computer-readablemedia may include hard drives, CDs, DVDs, print-outs, etc. In someembodiments, raw analog data from streamers may be stored as ageophysical data product. In other instances, the data may first bedigitized and/or conditioned prior to being stored as the geophysicaldata product. In yet other instances, the data may be fully processedinto a two- or three-dimensional map of the various geophysicalstructures before being stored in the geophysical data product. Thegeophysical data product may be produced offshore (e.g., by equipment ona vessel) or onshore (e.g., at a facility on land) either within theUnited States or in another country. If the geophysical data product isproduced offshore or in another country, it may be imported onshore to afacility in the United States. Once onshore in the United States,geophysical analysis may be performed on the geophysical data product.

In some instances of a typical marine seismic survey, one or more marineseismic sources may be used to generate seismic energy. The seismicenergy may travel downward through the water and through the formationsunderlying the water bottom. Impedance boundaries within the subseaformations may reflect (at least in part) the seismic energy thattravels through such formations. The reflected seismic energy may thentravel upwards. Geophysical sensors (e.g., hydrophones, geophones,accelerometers, etc.) may capture such reflected seismic energy. Thesegeophysical sensors may convert the captured seismic energy intosignals, such as optical or electrical signals. The electrical oroptical signals may then be interpreted to provide information as to thecomposition and/or structure of the various subsurface formations underthe water bottom. Such information may be used, for example, todetermine the possibility that such formations may contain mineraldeposits including hydrocarbons.

One type of marine seismic source that may be used in seismic surveyingoperations is a flexural disc projector, commonly known as a “bender.” Atypical bender may employ one or more piezoelectric elements, such thatthe mechanical vibration of the bender is driven by piezoelectricdistortion based on electrical energy applied to the piezoelectricelement. For example, when electrical energy (e.g., a voltage) isapplied to a piezoelectric material, a volume or length of thepiezoelectric element may increase or decrease in response. Thisphenomenon is generally termed the “inverse piezoelectric effect,” andit may be exploited in order to produce seismic energy. For example, apiezoelectric element may be attached (e.g., glued) to a portion of amarine seismic source, such that the contractions and/or expansions ofthe piezoelectric element may cause deformations in the portion, and thedeformations may cause seismic energy to be transmitted through thewater.

However, although common, typical piezoelectric benders may be subjectto various disadvantages in some cases. For example, they are oftenlabor-intensive and expensive to produce. Further, large piezoelectricsmay be especially difficult to produce, which can effectively limit thesize of piezoelectric benders. Piezoelectric elements may also be proneto cracking, as well as being sensitive to pressure variations.Accordingly, as described more fully below, some embodiments of thisdisclosure include marine seismic sources that use the Lorentz force(e.g., the force between a magnet and a current loop) to produce seismicenergy. This new technique may ameliorate some of the difficulties thathave arisen with piezoelectric marine seismic sources.

When seismic energy travels through water and subsea formations, higherfrequency seismic energy may typically be attenuated more rapidly thanlower frequency waves. Accordingly, seismic energy at lower frequencies(for example, infrasound) may typically be transmitted over longerdistances through water and subsea formations than higher frequencyseismic energy. A marine seismic source that produces seismic energy atlower frequencies may thus have utility in marine seismic surveys,particularly those conducted in increased water depths.

The design parameters for a marine seismic source may include, amongothers, seismic energy output within a low frequency range (e.g.,between 2 Hz and 20 Hz, or between 2 Hz and 10 Hz), water depth, weight,size and cost. For example, it may be advantageous for a marine seismicsource to be capable of supplying broadband low-frequency energy, e.g.,such that the frequency spectrum may be changed as desired according tothe geophysical properties in a survey. As modern marine seismic surveyscontinue to explore waters at increased depths, there is a continuingneed for a low cost (for example, both in terms of manufacturing costand operating cost) marine seismic source that produces high seismicenergy at lower frequencies, particularly at frequencies in the range ofapproximately 2 Hz to 10 Hz. Some embodiments of this disclosure areparticularly applicable in this frequency range, although otherembodiments are operable in larger ranges, such as 2 Hz to 20 kHz.

In the context of a low-frequency source (which generally refers to asource outputting a frequency in the range of 2-10 Hz), it may beadvantageous to tow the source at a depth of approximately λ/4, where λis the wavelength of the seismic energy being output by thelow-frequency source. For example, since seawater has a sound velocityof approximately 1,500 meters/second, λ/4 for a 5 Hz source would beapproximately 75 meters. For a frequency in the range of 2-10 Hz, λ/4would be between approximately 35 and 190 meters.

Turning now to FIG. 1, an exemplary illustration of marine seismicsource 100 is shown. As shown, marine seismic source 100 may be in theform of an electromechanical device such as a bender. It should be notedthat FIG. 1 may not necessarily be drawn to scale: in some embodiments,marine seismic source may have a larger or a smaller height relative toits radius.

Marine seismic source 100 in this embodiment is generally cylindrical,and it is arranged about axis 102. Marine seismic source 100 includestop plate 104, as well as a corresponding bottom plate 150 (not showndue to the perspective of FIG. 1). These plates are held in place viafasteners 106 (such as bolts), which connect them to hoop 108. Hoop 108extends around the circumference of marine seismic source 100 andprovides a desired separation between top plate 104 and bottom plate150. Hoop 108 may also provide a fixed contact against which top plate104 and bottom plate 150 can flex inwardly and outwardly.

One of ordinary skill in the art with the benefit of this disclosurewill understand that the references to “top” and “bottom” plate are notintended to indicate that the top plate must necessarily be above thebottom plate. These designations are instead intended only to simplifythe descriptions of the drawings in this disclosure. In embodiments ofmarine seismic sources according to this disclosure, the “top” and“bottom” plates may be in any desired physical orientation relative toone another.

An interior cavity may be formed between top plate 104 and bottom plate150, and this interior cavity may be configured to contain a volume ofgas (such as air, nitrogen, or any other suitable substance). In someembodiments, the volume of gas within the interior cavity may bepressurized. In marine applications, pressurizing and maintaining thevolume of gas within the interior cavity at an ambient hydrostaticpressure at a selected operating water depth may protect marine seismicsource 100 from collapsing from the ambient hydrostatic pressure.

Plates 104 and 150 may each be of a circular shape, an elliptical shape,or some other shape. Plates 104 and 150 may be made from a material suchas metal (e.g., aluminum, steel, etc.). Plates 104 and 150 may be madefrom the same material in one embodiment. Yet in another embodiment,they may be made from different materials.

Not shown in FIG. 1, various types of circuitry and components may beused to produce seismic energy by causing top plate 104 and/or bottomplate 150 to move. As noted above, one possibility is the use of one ormore piezoelectric elements (e.g., piezoelectric ceramics glued to theinterior or exterior surfaces of the plates). In accordance with thisdisclosure, however, magnetic elements and wire coils may in someembodiments also be used to cause marine seismic source 100 to produceseismic energy.

Turning now to FIGS. 2A and 2B, views are shown respectively of topplate 204 and bottom plate 250 of marine seismic source 200. A hoopoperable to connect these two plates (e.g., corresponding to hoop 108 inFIG. 1) is not shown, and axis 202 is shown as passing through both topplate 204 and bottom plate 250. When assembled, top plate 204 and bottomplate 250 would be aligned along axis 202. Top plate 204 includesmagnets 210 arranged in a circle. In some embodiments, magnets 210 maybe attached to the bottom surface of top plate 204. In otherembodiments, magnets 210 may extend through the interior of top plate204, being magnetically accessible to the bottom surface of top plate204. Magnets 210 may be any suitable type of magnet, such as permanentmagnets (e.g., made from any of various suitable materials) and/orelectromagnets.

Magnets 210 are shown as having their south poles pointed toward thecenter of top plate 204 and having their north poles pointed outward,but one of ordinary skill in the art with the benefit of this disclosurewill understand that this could be reversed in some embodiments. It may,however, be advantageous for all of magnets 210 to be facing in the samedirection (e.g., south poles all facing inward or all facing outward).

Further, magnets 210 need not necessarily be arranged in a circle. Acircular arrangement may be advantageous, for example, in a marineseismic source having circular plates. However, any suitable arrangementof magnets 210 may be used.

Bottom plate 250 is shown with wire coil 260 positioned such that itwould be adjacent to magnets 210 in the assembled device. In someembodiments, wire coil 260 may be attached to the top surface of bottomplate 250. In other embodiments, wire coil 260 may extend through theinterior of bottom plate 250, being magnetically accessible to the topsurface of bottom plate 250. It may be advantageous for magnets 210 andwire coil 260 to be placed such that, when electrical current is passedthrough wire coil 260 in the assembled device, an induced magnetic fieldaround wire coil 260 may interact with the magnetic field of magnets 210to cause the plates to move due to the Lorentz force. Wire coil 260 mayconsist of a single loop of wire or multiple loops of wire, asappropriate. Various electronic components, not shown in FIGS. 2A and2B, may be used to supply electrical energy to wire coil 260.

In the embodiment illustrated by FIGS. 2A and 2B, when electrical energy(e.g., a sinusoidal voltage or current) is applied to wire coil 260, topplate 204 and/or bottom plate 250 may bend, flex, or otherwise bedistorted (e.g., due to the Lorentz force arising from the interactionbetween the magnetic fields of magnets 210 and the induced magneticfield of wire coil 260), resulting in vibration and seismic energyoutput. In this embodiment, the plates may bend, flex, or otherwise bedistorted axially along axis 202. In some embodiments, the distortionsmay be axially symmetric or symmetric with respect to axis 202.According to one embodiment, a typical driving current may beapproximately 5 A/mm², and/or a typical driving voltage may be in therange of 0V-1 kV.

A maximum sound pressure level of a marine seismic source may typicallyoccur at or near a mechanical resonance frequency of the marine seismicsource. Broadly speaking, sound pressure level is a difference, in agiven medium, between a pressure associated with the seismic energy andan average local pressure. The square of this difference may be averagedover time and/or space, and a square root of this average may provide aroot-mean-square (RMS) pressure value or P_(rms). Sound pressure levelis a logarithmic measure indicating a ratio of a given P_(rms) relativeto a reference sound pressure or P_(ref). Sound pressure level istypically measured in decibels (dB). In marine applications, a referencepressure P_(ref) is usually 1 micropascal (1 μPa). In mathematicalterms, sound pressure level may be calculated by the equation below:Sound Pressure Level (dB)=20 log(P _(rms) /P _(ref))

One of ordinary skill in the art with the benefit of this disclosurewill understand that the diameter, thickness, and material constructionof top plate 204 and bottom plate 250 may advantageously be designed inorder to achieve desirable operational characteristics (e.g., aresonance frequency in a selected range, such as 2-20 Hz). This mayprovide a sufficiently large sound pressure level in the frequency rangeof interest. According to one embodiment, top plate 204 and bottom plate250 may be approximately 450-700 mm in diameter. According to oneembodiment, top plate 204 and bottom plate 250 may be approximately 3-5mm in thickness. Any suitable material may be used, such as aluminum,steel, other metals, alloys, composites, etc.

Magnets 210 and/or wire coil 260 may in some embodiments be attached byan adhesive, such as epoxy, or they may be bolted in place, or they maybe attached via any other suitable means. However, there is nolimitation as to how such components may be attached to plates 204 and250. As used herein, the words “attach,” or “attached” (and otherderivations thereof) should be understood to mean a connection betweencomponents, whether direct or indirect.

As will be described in more detail below, in some embodiments, morethan one wire coil 260 may be used; additionally, more than one set ofmagnets 210 may also be used. For example, a plurality of wire coils anda corresponding plurality of sets of magnets may in some embodiments bearranged in concentric circles on one or both of the plates.

Turning now to FIG. 3, a cross-sectional view of an embodiment of marineseismic source 300 is shown. Marine seismic source 300 is arranged aboutaxis 302, and it includes top plate 304 and bottom plate 350 separatedby hoop 308. In this illustration, top plate 304 has attached thereto aplurality of sets of magnets 310. That is, this cross-sectional viewdepicts a situation in which four concentric circles of magnets 310 havebeen attached to top plate 304. Four corresponding wire coils 360 havebeen attached to bottom plate 350.

In some embodiments, such as the embodiment shown in FIG. 3, magnets 310may have pole pieces 312 (e.g., made of iron or some other ferromagneticmaterial) attached thereto in order to concentrate their respectivemagnetic fields in a desired region. As shown, magnets 310 and polepieces 312 are arranged such that their magnetic fields are concentratedin a region proximate to wire coils 360. FIG. 3 is not necessarily drawnto scale, and one of ordinary skill in the art with the benefit of thisdisclosure will recognize that the spacing between pole pieces 312(and/or magnets 310) and wire coils 360 may advantageously be selectedin order to produce the desired forces. Further, pole pieces 312 may insome embodiments advantageously be designed such that they do not comeinto physical contact with wire coils 360 during normal operation ofmarine seismic source 300. It may further be advantageous for magnets310 not to come into contact with wire coils 360, and/or for pole pieces312 not to come into contact with bottom plate 350.

Turning now to FIG. 4, a cross-sectional detail view of a magnet thatmay be used in accordance with this disclosure is shown. Magnet 410 isin this embodiment a permanent magnet (e.g., made of a ferromagneticsubstance). Magnet 410 has pole pieces 412 attached to its oppositepoles. As shown by magnetic field lines 414, pole pieces 412 may have atendency to concentrate the magnetic field of magnet 410 in a desiredregion. In particular, in the arrangement shown in FIG. 4, the field isconcentrated most strongly in region 416 directly between pole pieces412. For this reason, pole pieces may be advantageously used in someembodiments of marine seismic sources according to the presentdisclosure in order to increase the Lorentz force between the magnetsand the wire coils. For example, region 416 may in some instances beplaced as near as is practicable to the corresponding wire coil in orderto increase the Lorentz force therebetween. In some embodiments, thecorresponding wire coil may even pass through region 416.

Turning now to FIGS. 5A and 5B, another embodiment of a marine seismicsource according to this disclosure. Marine seismic source 500 includestop plate 504 and bottom plate 550. Similar to FIGS. 2A and 2B, marineseismic source 500 is designed such that top plate 504 and bottom plate550 would be aligned along axis 502 in the assembled apparatus.

Due (for example) to differences in mass between magnets and wire coils,it may sometimes be the case that the combined mass of one plate (thatis, the mass of the plate plus the mass of any magnets and/or wire coilsattached thereto) is larger than the combined mass of the oppositeplate. In some cases, however, it may be advantageous for the plates ofa marine seismic source according to this disclosure to have equal mass(or approximately equal mass, such as within 1% of one another, within2% of one another, within 3% of one another, within 4% of one another,within 5% of one another, within 10% of one another, etc.). Equalizingthe mass of the two plates may have advantageous effects on the soundpressure level, the resonance frequency, the frequency spectrum, and/orother characteristics of some embodiments of marine seismic sourcesaccording to this disclosure.

Although mass equalization may in some instances be achieved by simplyadding mass to the lighter of the plates, it may also be achieved insome cases by arranging a marine seismic source according to thisdisclosure such that a first portion of the plurality of magnets isattached to the top plate and a second portion of the plurality ofmagnets is attached to the bottom plate, instead having all of themagnets attached to one plate, and all of the wire coils attached to theother plate. One example of such an embodiment is marine seismic source500.

As shown, top plate 504 includes wire coil 506, magnet set 508, wirecoil 510, and magnet set 512. Bottom plate 550 includes magnet set 552,wire coil 554, magnet set 556, and wire coil 558. In this embodiment,the magnet sets and wire coils are arranged such that, in the assembledapparatus, wire coil 506 would correspond to magnet set 552, wire coil554 would correspond to magnet set 508, wire coil 510 would correspondto magnet set 556, and wire coil 558 would correspond to magnet set 512.One of ordinary skill in the art with the benefit of this disclosurewill understand that any suitable number of wire coils and magnet setsmay be used in a particular embodiment. It should be noted that whilemarine seismic source 500 shows an arrangement in which the respectivewire coils and magnet sets are alternated with one another, this is notnecessary. The wire coils and magnet sets may be arranged in anysuitable configuration.

For the sake of simplicity in FIGS. 5A and 5B, the polarities of themagnets are not shown. One of ordinary skill in the art with the benefitof this disclosure will understand that it may be advantageous for themagnets within a particular set all to have the same polarity in orderto increase the total Lorentz force. However, this is not a requirement,and it is also not a requirement that the polarity of a particular setof magnets within marine seismic source 500 have the same polarity asthe other sets.

For example, magnet sets 508 and 512 may or may not have the samepolarity, magnet sets 508 and 552 may or may not have the same polarity,etc. The circuitry that controls wire coils 506, 510, 554, and 558 mayin some instances take into account the polarity of the correspondingmagnet set. So for example, one particular wire coil may be driven witha sinusoidal voltage, and a different wire coil may be driven with asinusoidal voltage having a 180° phase offset, as appropriate based onthe polarity of the corresponding magnet sets.

Turning now to FIG. 6, an embodiment is illustrated in which an array ofseveral individual marine seismic sources 620 may be arranged in ahousing into stack assembly 600. Although the individual marine seismicsources 620 are capable of providing seismic energy, it may beadvantageous to combine them in some embodiments into a stack such asstack assembly 600. Such an arrangement may in some instances be used toincrease the total sound pressure level achievable, relative to what maybe achievable via a single marine seismic source 620. In one embodiment,stack assembly 600 may include marine seismic sources 620 in a seriesconfiguration. In other embodiments, parallel configurations arepossible, as well as embodiments that incorporate both series andparallel components. Stack assembly 600 may include top plate 604 andbottom plate 650 to which marine seismic sources 620 may be secured.

FIG. 7 illustrates yet another embodiment of a stack assembly in whichstack assembly 700 additionally includes boot assembly 702. Bootassembly 702 may enclose and/or be disposed around the individual marineseismic sources shown in FIG. 6, for example. In one particularembodiment, boot assembly 702 may include a liquid (in some cases, anelectrically insulating material such as an electrically insulating oilmay be used) in which the individual marine seismic sources may beimmersed or at least partially disposed. This liquid may serve as anadditional protective layer for the individual marine seismic sources,and it may also provide a medium through which seismic energy istransmitted.

Turning now to FIG. 8, flow diagrams are presented in accordance withaspects of this disclosure. It should be understood that in someembodiments, fewer than all steps of a particular process flow may beperformed in accordance with this disclosure. Further, it should beunderstood that in some embodiments, steps may be performed in adifferent order.

FIG. 8 is a flow diagram illustrating an embodiment of a method 800 ofoperating an apparatus according to this disclosure. Flow begins atblock 810, in which a marine seismic source is actuated. The marineseismic source includes a first plate and a second plate configured tomove relative to one another to produce seismic energy, a plurality ofmagnets disposed between the first and second plates, and a plurality ofwire coils disposed between the first and second plates. The seismicenergy may be transmitted from the marine seismic source into asurrounding medium, such as seawater. The magnets and wire coils may bedisposed in any suitable configuration. Certain examples of suitableconfigurations have been described above. For example, in someinstances, all of the magnets may be disposed on the first plate, andall of the wire coils may be disposed on the second plate (or viceversa). In other instances, each plate may include some magnets and somewire coils.

In some instances, the marine seismic source may be a single source(e.g., an individual bender). In other instances, it may be an array ofsuch single sources. If the marine seismic source includes an array ofsingle sources, then it is contemplated that more than one source in thearray of sources may include first and second plates. For example, eachsource in the array may be substantially similar to the others in thearray. For example, each source in the array may include first andsecond plates. Flow proceeds to block 820.

At block 820, the seismic energy is detected. This may be accomplishedvia any of various types of sensors, such as hydrophones, geophones,accelerometers, etc. Further, the seismic energy may in some embodimentsbe detected after it has interacted with various subsea formations. Flowends at block 820.

Although specific embodiments have been described above, theseembodiments are not intended to limit the scope of the presentdisclosure, even where only a single embodiment is described withrespect to a particular feature. Examples of features provided in thedisclosure are intended to be illustrative rather than restrictiveunless stated otherwise. The above description is intended to cover suchalternatives, modifications, and equivalents as would be apparent to aperson skilled in the art having the benefit of this disclosure.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Accordingly, new claims may be formulatedduring prosecution of this application (or an application claimingpriority thereto) to any such combination of features. In particular,with reference to the appended claims, features from dependent claimsmay be combined with those of the independent claims and features fromrespective independent claims may be combined in any appropriate mannerand not merely in the specific combinations enumerated in the appendedclaims.

What is claimed is:
 1. An apparatus, comprising: a marine seismic sourcethat includes: a first plate that defines a first central axis and afirst outer edge, the first plate exposed on an exterior surface of themarine seismic source; a second plate that defines a second central axisand a second outer edge, the second central axis coaxial with the firstcentral axis, the second plate parallel to the first plate, and thesecond plate exposed on an exterior surface of the marine seismicsource; a hoop disposed between the first and second plates at the outeredges of the first and second plates; an interior cavity defined betweenthe first plate, the second plate, and the hoop; a plurality of magnetsdisposed within the interior cavity; and a plurality of wire coilsdisposed within the interior cavity; wherein, in response to an electriccurrent being passed through the plurality of wire coils, the first andsecond plates are configured to flex inwardly and outwardly relative tothe hoop along the coaxial first and second central axis.
 2. Theapparatus of claim 1, wherein the marine seismic source is configured toproduce seismic energy within a frequency range of 2 Hz to 20 Hz.
 3. Theapparatus of claim 2, wherein the seismic energy is within a frequencyrange of 2 Hz to 10 Hz.
 4. The apparatus of claim 1, wherein theplurality of magnets are attached to the first plate, and wherein theplurality of wire coils are attached to the second plate.
 5. Theapparatus of claim 1, wherein a first portion of the plurality ofmagnets is attached to the first plate, and wherein a second portion ofthe plurality of magnets is attached to the second plate.
 6. Theapparatus of claim 5, wherein a first portion of the plurality of wirecoils corresponding to the second portion of the plurality of magnets isattached to the first plate, and wherein a second portion of theplurality of wire coils corresponding to the first portion of theplurality of magnets is attached to the second plate.
 7. An apparatus,comprising: a marine seismic source that includes: a first plate; asecond plate; a plurality of magnets disposed between the first andsecond plates; and a plurality of wire coils disposed between the firstand second plates; wherein, in response to an electric current beingpassed through the plurality of wire coils, the first and second platesare operable to move relative to one another; wherein a first portion ofthe plurality of wire coils corresponding to the second portion of theplurality of magnets is attached to the first plate, and wherein asecond portion of the plurality of wire coils corresponding to the firstportion of the plurality of magnets is attached to the second plate; andwherein a first combined mass of the first plate, the first plurality ofmagnets, and the first plurality of wire coils is within 5% of a secondcombined mass of the second plate, the second plurality of magnets, andthe second plurality of wire coils.
 8. The apparatus of claim 1, whereinthe plurality of magnets includes pole pieces operable to increase amagnetic field at the plurality of wire coils.
 9. The apparatus of claim1, wherein the plurality of magnets is a plurality of permanent magnets.10. A marine seismic source, comprising: a housing; and a plurality ofbenders disposed within the housing, wherein each bender in theplurality of benders includes: a first plate that defines a firstcentral axis and a first outer edge, the first plate exposed on anexterior surface of the marine seismic source; a second plate thatdefines a second central axis and a second outer edge, the secondcentral axis coaxial with the first central axis, the second plateparallel to the first plate, and the second plate exposed on an exteriorsurface of the marine seismic source; a hoop disposed between the firstand second plates at the outer edges of the first and second plates; aninterior cavity defined between the first plate, the second plate, andthe hoop: a plurality of magnets disposed within the interior cavity;and a plurality of wire coils disposed within the interior cavity;wherein the first and second plates are configured to flex inwardly andoutwardly relative to the hoop along the coaxial first and secondcentral axis.
 11. The marine seismic source of claim 10, wherein, for atleast one bender in the plurality of benders, the plurality of magnetsis attached to the first plate, and the plurality of wire coils isattached to the second plate.
 12. The marine seismic source of claim 10,wherein, for at least one bender in the plurality of benders: a firstportion of the plurality of magnets is attached to the first plate; asecond portion of the plurality of magnets is attached to the secondplate; a first portion of the plurality of wire coils corresponding tothe second portion of the plurality of magnets is attached to the firstplate; and a second portion of the plurality of wire coils correspondingto the first portion of the plurality of magnets is attached to thesecond plate.
 13. The marine seismic source of claim 12, wherein thefirst portion of the plurality of magnets and the first portion of theplurality of wire coils are arranged alternatingly on the first plate.14. The marine seismic source of claim 10, wherein, for at least onebender in the plurality of benders, at least one magnet in the pluralityof magnets includes two pole pieces.
 15. The apparatus of claim 1:wherein the plurality of magnets further comprises a first plurality ofmagnets within the interior cavity and connected to the first plate; andwherein the plurality of wire coils further comprises a first pluralityof wire coils within the interior cavity and connected to the secondplate; wherein the first plurality of magnets are arranged such thatmagnetic fields of the first plurality of magnets are concentrated atthe first plurality of wire coils.
 16. The apparatus of claim 15,further comprising: a first plurality of pole pieces connected to atleast some of the first plurality of magnets.
 17. The apparatus of claim15, further comprising: a second plurality of magnets within theinterior cavity and connected to the second plate; and a secondplurality of wire coils within the interior cavity and connected to thefirst plate; wherein the second plurality of magnets are arranged suchthat magnetic fields of the second plurality of magnets are concentratedat the second plurality of wire coils.